1. Field of the Invention
The present invention relates to a semiconductor device in which a depletion type metal-oxide-semiconductor (MOS) transistor and an enhancement type MOS transistor are employed.
2. Description of the Related Art
In recent years, along with progress in technology for reducing a chip size of an integrated circuit (IC), leveling of the technology has been advanced. One way to maintain advantage in the current technology progress is to create a high-value-added product having superior characteristics.
The above-mentioned characteristics may be of various types, and include a temperature characteristic and sensitivity. First, the temperature characteristic of the IC is described as an example. In order to improve the temperature characteristic of the IC, the stability of a reference voltage, which exists in the IC, to temperature is mainly required.
For a reference voltage generating circuit, various types of circuits have been considered in accordance with a manufacturing process or layout in the IC. One of the most generally employed circuits with convenience may be a combination of a depletion type MOS transistor and an enhancement type MOS transistor.
With regard to the reference voltage circuit in which the depletion type MOS transistor and the enhancement type MOS transistor are employed, a number of ideas for improving the temperature characteristic through an inventive approach to the circuit have been described in documents (see, for example, JP 05-289760 A and JP 11-134051 A).
Next, in terms of the sensitivity, in a case of a voltage regulator, an output voltage should be stable against an input voltage. In this case, a reference voltage or other analog circuit characteristics are required to be stable and not to change depending on the input voltage. Also in this case, through the inventive approach to the circuit, suppression of the fluctuations of the reference voltage and the like are generally attempted.
However, in the above-mentioned methods the inventive approach to the circuit itself makes a circuit structure complicated and increases an area of the IC. At present, costs of the IC are increasingly reduced, and downsizing of the IC is also an important matter. Accordingly, an increase in the area of the IC by the circuit provided for adding a value is not effective.
Further, in a case where a production of an existing product is shifted to another factory for manufacture, a significant load is imposed for redoing a circuit design or layout when the product shows different characteristics.